Force Imaging
Force Calibration (Contact Mode AFM)
192
MultiMode SPM Instruction Manual
Rev. B
3. In the
Channel 1
panel select
Deflection
as the
Data Type
.
4. Maximize the
Data Scale
parameter.
5. Adjust the
Ramp size
parameter to about
1µm
.
6. If the tip does not reach the sample surface (for example, see
and 3), slowly increase the
Z scan start
value.
7. As the
Z scan start
increases, the traces on the force curve move to the right.
8. Adjust the
Setpoint
parameter.
Note:
Adjusting the
Setpoint
value moves traces up and down on the graph.
Decreasing the value moves the curve up while increasing the value moves the
curve down. (Remember that the green horizontal centerline of the graph is
equal to the
Setpoint
if the
Data Center
=
0
, so that changing the
Setpoint
actually shifts the graph up and down.) Typically, changing the
Setpoint
value
to -
2 volts
shows the desired features of the force curve. If it does not, decrease
the
Setpoint
value further.
Note:
Sometimes the entire ramp will occur in the contact portion of the curve (see
, between points 3 and 5). If this occurs, increase the
Ramp size
and/or decrease the
Z scan start
.
11.4.2 Helpful Suggestions
To minimize or calculate the contact force between the tip and sample, it is important to obtain a
good force curve; however, there will be situations where you will not obtain a good force curve no
matter how much you adjust the
Ramp size
,
Z scan start
and
Setpoint
. The following sections
discuss a few such situations. Suggestions on working in
Force Calibration
mode are included.
False Engagement
illustrates a force curve resulting from a falsely engaged tip. The photodiode receives
light re
fl
ected off the sample, causing an increase in the de
fl
ection signal until the signal equals the
setpoint and the system “engages” (even though the tip is not on the surface). Interference in the
re
fl
ected light causes the hump-shaped waveform.
The easiest way to correct a false engagement is to withdraw the tip, adjust the photodetector
positioner to make the top/bottom differential voltage more negative, then re-engage the tip. This
compensates for the slight slope in
).
